The invention is based on a piston pump and a brake system having a piston pump.
The piston pump is intended in particular as a pump in a brake system of a vehicle and is used for controlling the pressure in the wheel brake cylinders. Depending on the type of brake system, the abbreviations ABS, ASR, FDR and EHB are used for such brake systems. In the brake system, the pump serves for instance to return brake fluid from one or more wheel brake cylinders to a master cylinder (ABS) and/or to pump brake fluid out of a supply container into one or more wheel brake cylinders (ASR or FDR or EHB). In a brake system with wheel slip control (ABS or ASR) and/or a brake system serving as a steering aid (FDR) and/or an electrohydraulic brake system (EHB), the pump is needed. With wheel slip control (ABS or ASR), for instance, locking of the wheels of the vehicle during a braking event involving strong pressure on the brake pedal (ABS) and/or spinning of the driven wheels of the vehicle in the event of strong pressure on the gas pedal (ASR) can be prevented. In a brake system serving as a steering aid (FDR), a brake pressure is built up in one or more wheel brake cylinders independently of an actuation of the brake pedal or gas pedal, for instance to prevent the vehicle from shifting out of the lane desired by the driver. The pump can also be used in an electrohydraulic brake system (EHB), in which the pump pumps the brake fluid into the wheel brake cylinder or wheel brake cylinders if an electric brake pedal sensor detects an actuation of the brake pedal, or in which the pump is used to fill a reservoir of the brake system.
What is important is that in the brake system, the pressure in the individual brake cylinders can be controlled independently of the pressure in the other brake cylinders. Therefore in a vehicle with four brake cylinders, typically piston pumps are provided in which each of the brake cylinders is assigned its own pump element each with one pump piston. Along with this, U.S. Pat. No 4,875,741 shows a brake system in which two brake cylinders are connected to one pump element. However, so that the pressure medium cannot overflow unintentionally from one of the brake cylinders into the other brake cylinder in this version, a pump element with two work pressure chambers is used here. One of the two work pressure chambers is located on the face end on the pump piston, and the other work pressure chamber is formed as an annular chamber by providing the pump piston with a shoulder. In the brake system shown in U.S. Pat. No. 4,875,741, one inlet-side check valve and one outlet-side check valve is provided in the piston pump for each of the brake cylinders. While the pump piston is forced into the housing of the piston pump by the eccentric element, the pressure medium is forced out of the face-end work pressure chamber and at the same time out of the annular work pressure chamber into an outlet connection, each via a respective outlet valve. When the pump piston then moves out of the housing again, both work pressure chambers increase in size, and the pump element aspirates the pressure medium into both work pressure chambers. As a result, the pump operates very unevenly, creating major pressure pulsations and hence is very noisy.
The piston pump of the invention and the brake system set forth have the advantage over the prior art that upon motion of the pump piston, the feeding of the pressure medium from the two work pressure chambers is done in phase-offset fashion, thus making the pumping of pressure medium substantially more uniform, which leads to a considerable reduction in noise. Because feeding is more uniform, the maximum flow speed of the pressure medium is also less, and as a result the efficiency of the piston pump is improved, or else smaller line cross sections can be selected.
For safety reasons, two brake circuits are often provided in brake systems. For safety reasons, it is important that the two brake circuits be cleanly separated from one another. One substantial advantage of the piston pump proposed is that the individual brake circuits of the vehicle brake system can be cleanly separated from one another.
One particular advantage is that the feed flow can be made more uniform despite low effort and expense for drilling inside the housing of the piston pump.
Advantageous refinements of and improvements to the piston pump and the brake system are possible with the provisions recited hereinafter.
If the transmission mechanism or the eccentric element between the drive motor and the pump piston is embodied such that compressive and tensile forces can be transmitted by the drive motor to the outlet connection, this offers the advantage of being able to dispense with a large-sized spring to restore the pump piston.